In recent years, research and development have been extensively carried out on a light-emitting element which uses an organic compound having a light-emitting property. In a basic structure of such a light-emitting element, a layer which contains an organic compound having a light-emitting property is sandwiched between a pair of electrodes. By applying voltage to this element, electrons and holes are injected from the pair of electrodes respectively into the layer containing an organic compound having a light-emitting property; thus, current flows. Then, by recombination of those carries (electrons and holes), the organic compound having a light-emitting property forms an excited state and emits light when the excited state returns to a ground state. Because of such a mechanism, this light-emitting element is called a light-emitting element of a current excitation type.
The excited state of the organic compound includes a singlet-excited state and a triplet-excited state, and light emission from a singlet-excited state is called fluorescence and light emission from a triplet-excited state is called phosphorescence. Such a light-emitting element can be manufactured generally by using an organic thin film with a thickness of about 0.1 μm; therefore, the light-emitting element has advantages of thinness and lightness in weight. Moreover, since it takes about 1 μs or shorter from carrier injection until light emission, very high-speed response is also one of its advantages. Because of these advantages, the light-emitting element is considered to be preferable as a flat-panel display element.
Since the light-emitting element is formed into a film shape, surface light emission can be easily obtained by forming a large-sized element. This is a feature which is difficult to be obtained in point light sources typified by an incandescent lamp and an LED or line light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is also effectively used as a surface light source applicable to illumination and the like.
In display devices to be incorporated into various information processing appliances which have rapidly developed recently, there is a demand particularly for lower power consumption, and decrease in drive voltage for light-emitting elements has been attempted in order to fulfill this demand.
For example, according to Reference 1 (Japanese Patent Application Laid-Open No. 2005-123095), an effect of decreasing drive voltage is obtained by using a layer in which vanadium oxide and α-NPD are co-evaporated.
However, the layer in which vanadium oxide and α-NPD are co-evaporated has a peak of an absorption spectrum in 400 to 800 nm, which is a visible region. In particular, the layer has a high peak of absorption in about 150 to 500 nm, which is a blue region. Therefore, light emitted from a light-emitting substance is absorbed in the layer in which vanadium oxide and α-NPD are co-evaporated, which leads to a problem in that external extraction efficiency of light emission is lowered.